1. Field of the Invention
The present invention relates to a communication control technique that controls multiplex communication in the form of fixed-length cells. More specifically the present invention pertains to a buffer management technique to manage cells stored in a cell buffer assembly.
2. Description of the Related Art
A known technique of high-speed transfer of large-capacity data, such as movie data, is asynchronous transfer mode (ATM). ATM time division data, for example, voice data, static image data, or movie data, into fixed-length, 53-byte cells and carries out communication in the form of such cells. Transmission of the time-divided cells enables signals from a plurality of input ports to be multiplexed. The cells are temporarily stored in a buffer included in a communication controller or a multiplexer, and are read from the buffer at a desired rate or at desired time intervals set for each communication.
There is a known technique for sharing the buffer among all the input ports. This method manages the place and the order of storage of cells at each input port and shares the storage area for cells.
FIG. 18 illustrates the configuration of a prior art communication control system. Cells input via ‘n’ input ports are stored in a common cell buffer 3, which is shared among all the input ports, under the control of a writing controller 1. A vacant buffer management memory 4 manages writable areas in the cell buffer 3. The writing controller 1 writes a cell in a vacant area specified by the vacant buffer management memory 4 and transmits information on a storage place of the cell to a class management memory 2. The class management memory 2 manages the place and the order of storage of cells at each input port.
A reading controller 6 reads cells from the cell buffer 3, multiplexes the cells, and outputs the multiplexed cells to the ATM network. The reading controller 6 refers to the class management memory 2 and specifies the order of reading the cells at each input port. The reading controller 6 reads and outputs the cells at each input port, which have been time-divided at a preset rate, in the specified order. The read-out area is specified as a vacant area and becomes subject to management by the vacant buffer management memory 4. The writing and reading operations are based on the time counted by a clock 5, in order to ensure the preset communication rate.
This prior art communication control technique does not require an individual cell buffer for each input port, thus desirably reducing the total capacity of the cell buffer. An increase in number of areas in the class management memory readily extends input ports without requiring any additional buffer.
In ATM, a minimum cell rate (MCR) is set for each input port. The transfer rate exceeding the MCR is allowed in the state of low traffic. Under the convergence of traffic, cells are discarded to ensure the MCRs of the other input ports. The prior art technique discards cells, based on the occupation rate of the storage area in the cell buffer. When the occupation rate in the cell buffer exceeds a preset threshold value under the convergence of traffic, cells stored at the transfer rate exceeding the MCR, that is, non-priority cells, are discarded.
On the management of the storage places of cells in the cell buffer, the prior art system carries out discard of non-priority cells as part of the cell reading control. Namely the reading controller discards a non-priority cell without transmission in the course of reading the cell from the cell buffer. The prior art system requires relatively complicated control procedures, since the reading controller performs the two different control processes, that is, the cell reading control and the cell discard control. This prior art technique reads the non-priority cell, irrespective of non-execution of transmission. This lengthens the total processing time. Discard of the non-priority cell is carried out on completion of the process of reading a normal cell to be transmitted. This lowers the efficiency of processing.